This invention relates to a novel method and a novel apparatus for depositing oxide-cathode precursor material on a cathode substrate by air spraying. The coated substrate is subsequently installed in a vacuum electron tube and then, by further processing, converted to an oxide-cathode.
An initial step in preparing an oxide cathode is to deposit a layer of precursor material, usually alkaline earth carbonates, on a cathode substrate, which usually consists essentially of a nickel alloy containing silicon and magnesium. Subsequently, the carbonates of the layer are converted to oxides by thermal decomposition. Then, the converted layer is activated; that is rendered electron emissive by further heat treatment, by which free barium metal is released from the oxides and associated donor centers are formed therein. In order to produce a cathode having optimum amounts of electron emission over a long emission life, it is important to deposit a precursor layer that is highly uniform with respect to porosity, weight and surface texture. The deposition process should produce precursor layers at low cost, be easy to control and be able to provide a high yield of useful cathodes.
Ordinarily, the precursor layer is deposited by air-spraying multiple sublayers of a slurry of the precursor material onto the substrate. The slurry of precursor material is sucked into the air stream in the spray gun through a slurry-supply tube from a pool of slurry in an open vessel. The slurry is drawn into the air stream at the nozzle of the gun by the Venturi effect which produces a siphon pressure in the slurry-supply tube. Because the siphon pressure is relatively low, the slurry-supply tube must have a short length and a relatively large cross-sectional area so that it offers a minimal resistance to the flow of a slurry therethrough.
It has been found that the siphon pressure has a constant pressure component and a fluctuating pressure component. The fluctuating pressure component can be detected at the nozzle of the gun and also at points in the slurry-supply tube all the way back to the slurry vessel. The fluctuating component is believed to be partly responsible for uncontrolled variations in the porosity, unit weight and surface texture produced in the air-sprayed precursor layers. The rate of flow of slurry in this prior method is ordinarily controlled by the position of a needle valve in the spray gun and is significantly affected by the drop in slurry level in the vessel during the spraying operation, and by fluctuations in the siphon pressure.